The present invention relates to an X-ray diagnostic apparatus wherein X-rays are applied to a subject, and a picture according to a transmitted X-ray dose is displayed on a monitor.
When imaging or photographing a desired portion of a subject, such as a living human body, i.e. patient, on an X-ray diagnostic apparatus, the prevailing method is such that the desired portion is image photographed continuously for a certain period of time as in the case of cinemato-graphing. The cinematographing is intended for observation of a contrast image, and an X-ray imaging or photographing system (hereinafter simply called imaging system) or the subject may be moved regardless of imaging process. For keeping a cinefilm density constant in this case, an outgoing light of I. I. (image intensifier) for inputting X-rays having passed through the subject is detected by a photoelectron multiplier and a detected result is fed back to a high voltage generator, thereby controlling X-ray imaging or photographing conditions (hereinafter simply called imaging conditions) such as tube voltage, tube current and the like. Further, the outgoing light of I. I. also changes when a visual field mode of I. I. is changed, therefore the imaging conditions are controlled according to a similar method.
On the other hand, DSA (Digital Subtraction Angiography) is employed as another imaging system on the X-ray diagnostic apparatus for the purpose of subtraction between picture images. To move the imaging system or subject during imaging process in the DSA as in the case of aforementioned cinematographing is to lead to an erroneous subtraction, thus an artifact may arise on a DSA image (subtraction image) to bring about a picture image improper to diagnosis. Further, to change the imaging conditions during imaging process is also to cause a change in density of the DSA image, and a diagnostic efficiency may deteriorate, which is not preferable. Accordingly, in the DSA optimum imaging conditions will be decided immediately before the start in the imaging process, which is then carried out with the imaging conditions being fixed thereto.
In recent years, meanwhile, a non-ionic contrast medium has been developed, and a dosage is less limited, therefore a contrast examination with the object of observing a contrast image in view without subtraction has increased also in DSA examination carried out on a digital image processing unit. As in the case of aforementioned cinematographing, a movement of the imaging system or subject and imaging conditions during the imaging process may be controlled. In case the imaging system and the subject are fixed while an observation of the contrast image is a principal object, there is a demand for utilizing the DSA image subjected to a subtraction process as diagnosis information under the state where the artifact and the change in density caused thereon are not to deteriorate severely a diagnostic efficiency.
Meanwhile, according to the prior art technology described above, there is a problem inherent in a prior art X-ray diagnostic apparatus such that if the imaging conditions are controlled whenever the DSA is carried out as mentioned, an outgoing light of I. I. changes according to an inflow of the contrast medium and thus the imaging conditions are changed. Besides, if the imaging conditions are fixed, the outgoing light of I. I. becomes excessively high or low in accordance with a movement of the imaging system or subject, therefore a density of the contrast image comes to change largely.